This is sockets/is_seqnum_v2_sv.c, an example to accompany the book, The Linux Programming Interface.

This file is not printed in the book; it is the solution to Exercise 59-2:b (page 1237).

The source code file is copyright 2022, Michael Kerrisk, and is licensed under the GNU General Public License, version 3.

In the listing below, the names of Linux system calls and C library functions are hyperlinked to manual pages from the Linux man-pages project, and the names of functions implemented in the book are hyperlinked to the implementations of those functions.


Download sockets/is_seqnum_v2_sv.c

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Function list (Bold in this list means a function is not static)

/* is_seqnum_v2_sv.c

   A simple Internet stream socket server. Our service is to provide unique
   sequence numbers to the client.

   This program is the same as is_seqnum_cl.c, except that it uses the functions
   in our inet_sockets.c library to simplify set up of the server's socket.

   Usage:  is_seqnum_sv [init-seq-num]  (default = 0)

   See also is_seqnum_v2_cl.c.
#include "is_seqnum_v2.h"
main(int argc, char *argv[])
    if (argc > 1 && strcmp(argv[1], "--help") == 0)
        usageErr("%s [init-seq-num]\n", argv[0]);

    uint32_t seqNum = (argc > 1) ? getInt(argv[1], 0, "init-seq-num") : 0;

    /* Ignore the SIGPIPE signal, so that we find out about broken connection
       errors via a failure from write(). */

    if (signal(SIGPIPE, SIG_IGN) == SIG_ERR)    errExit("signal");

    socklen_t addrlen;
    int lfd = inetListen(PORT_NUM_STR, 5, &addrlen);
    if (lfd == -1)
        fatal("inetListen() failed");

    /* Allocate a buffer large enough to hold the client's socket address */

    struct sockaddr *claddr = malloc(addrlen);
    if (claddr == NULL)

    for (;;) {                  /* Handle clients iteratively */

        /* Accept a client connection, obtaining client's address */

        socklen_t alen = addrlen;
        int cfd = accept(lfd, (struct sockaddr *) claddr, &alen);
        if (cfd == -1) {

        char addrStr[IS_ADDR_STR_LEN];
        printf("Connection from %s\n", inetAddressStr(claddr, alen,
                        addrStr, IS_ADDR_STR_LEN));

        /* Read client request, send sequence number back */

        char reqLenStr[INT_LEN];        /* Length of requested sequence */
        if (readLine(cfd, reqLenStr, INT_LEN) <= 0) {
            continue;                   /* Failed read; skip request */

        int reqLen = atoi(reqLenStr);
        if (reqLen <= 0) {              /* Watch for misbehaving clients */
            continue;                   /* Bad request; skip it */

        char seqNumStr[INT_LEN];        /* Start of granted sequence */
        snprintf(seqNumStr, INT_LEN, "%d\n", seqNum);
        if (write(cfd, seqNumStr, strlen(seqNumStr)) != strlen(seqNumStr))
            fprintf(stderr, "Error on write");

        seqNum += reqLen;               /* Update sequence number */

        if (close(cfd) == -1)           /* Close connection */


Download sockets/is_seqnum_v2_sv.c

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